Directed and focused laser energy is used for a variety of manufacturing tasks requiring precision material removal such as drilling of blind and through vias in electronic circuit substrates, repair or modification of semiconductor circuits, dicing or scribing of circuit assemblies for singulation, or other complex tasks involving drilling, cutting machining, or exposure of photosensitive materials. Materials process range from organic circuit substrate materials such as FR-4 or ABT, semiconductor wafers of silicon or sapphire, metals or metal foils or various types of plastic or glass. What these applications have in common is that the machining is accomplished by focusing a laser beam or laser pulses into a small focal spot proximate to the workpiece, thereby concentrating the laser energy into a focal spot which is imaged onto or near the surface of a workpiece in order to vaporize, ablate or otherwise cause the removal of material.
This type of laser focal spot machining is particularly useful in the manufacture of electronic substrates. The manufacturers of electrical and electro-optic assemblies continue to strive for higher density, faster circuitry and greater integration of components in order to deliver greater value to the consumer. As part of this striving, manufacturers are seeking to improve methods of interconnecting devices on substrates including electrical and electro-optic devices. Components are typically interconnected by attaching them to possibly multilayer substrates that have had circuit patterns applied to the surfaces of the substrates using additive or subtractive etching along with photosensitive resist applications.
Problems exist with this method of interconnecting electronic or electro-optic devices on a substrate. The first is that as circuit switching speeds increase, the electrical properties of the conductors on the substrate become a significant factor in limiting the speed at which the circuits can be clocked. Forming planar conductors on the surface of a substrate can exacerbate this problem. As currents and switching speeds increase, the shape of the cross-section of the conductor can become a significant factor in the electrical performance of the circuit in it occurs. In particular, changes in the cross-sectional shape of the conductor can cause unwanted changes in the conductor's impedance, which can cause reflections and signal loss. In addition, the density with which circuits can be applied to the substrate is partially a function of the size of conductors on the surface of the substrate.
For optical waveguides the cross-sectional topology of the channel or trench forming the waveguide is critical. In some cases, in addition to acting as a waveguide, parts of the channel can be used as optical elements such as mirrors. In this case the topology and the surface texture are critical elements of the channel feature in addition to the topology of the waveguide itself.
A prior art method of forming circuits interconnects on a substrate by laser machining trenches or channels in the surface of the substrate, sometimes referred to a laser direct ablation or LDA is given in U.S. Pat. No. 7,014,727 METHOD OF FORMING HIGH RESOLUTION ELECTRONIC CIRCUITS ON A SUBSTRATE, Christopher Wargo, et. al., inventors, describing a method of forming conductors on organic-based substrates such as FR-4. The method described uses a laser to machine channels in a layer of resist material applied to the surface of a substrate. The method also describes machining this channel into the substrate. These channels are subsequently filled with conducting material to form conductors. This patent discloses the need to form the channels in the substrate with appropriate size, shape and depth but does not disclose or discuss any particular way to achieve this. In particular this reference does not discuss maintaining the size and shape of channels as the path is changed in direction and shape.
In addition, channels in an electronic circuit substrate can also be used as optical waveguides. A description of optical waveguides is found in “Laser Ablation and Laser Direct Writing as Enabling Technologies for the Definition of Micro-Optical Elements”, by Nina Hendrickx, et. al., published in Integrated Optics: Theory and Applications, edited by Tadeusz Pustelny et. al., Proceedings of the SPIE Vol. 5956 pp 5961B-1-5961B-10. In this article, the authors describe using a laser to machine waveguides in substrates in order to integrate electro-optic components such as laser diodes with electronic components more closely. The article discusses the need to form waveguides with surface textures appropriate to optical devices and how it can be achieved, but does not discuss in detail how the shape and size of the waveguide can be controlled while laser machining.
A problem with laser machining channels to form conductors or waveguides in electronic substrates is that in general, these channels have to change direction on the surface of the substrate in order to connect desired points. This requires that the laser machine shapes such as curves in the surface of the substrate. Machining curves in the surface using prior art laser spots will cause the channel to vary in depth as it is machined. For example Gaussian profile beam will, on translation, leave a super-Gaussian profiled groove. A round-flat profile (Top Hat profile) beam will leave a cosine-shaped groove and a square-flat profile will leave a flat square groove. Current state of the art is to use a flat-square profile for operations like scribing or dicing where a uniform dose of laser irradiation is desired to leave a uniform straight-line cut with minimal edge effects. This is typically accomplished with refractive, diffractive, or holographic beam shapers that are placed in the beam path to transform by redistribution of energy an incident beam that is typically substantially Gaussian into one that is substantially uniform (round or square outline with substantially uniform intensity within).
This effect is illustrated in FIG. 1, which is a schematic diagram of a channel laser machined with a top hat profile beam. In FIG. 1, a channel 10 is laser machined into a substrate 12 with a pulsed laser beam (not shown) with a top hat or “round-flat” profile. The overlapped circles 14 represent positions of the laser pulses. When laser machining channels with a pulsed laser, typically the laser is indexed or moved smoothly and continuously along the path of the channel to be machined as the laser is pulsed, thereby machining a smooth and continuous feature in the substrate. The actual number of positions will vary depending upon the size of the laser spot, the desired width of the channel and the energy per pulse delivered to the substrate, and hence the amount of material removed per pulse. The number of pulse positions shown is much reduced from actual practice to show the positions more clearly. The amount of material removed is calculated from the cumulative dose received at each point in the channel from the multiple pulses each point receives as the pulsed beam is translated down the predetermined path that the channel will follow. All these examples would apply equally well if the laser were continuous wave (CW) rather than pulsed.
FIG. 2 shows a diagram of a rectangular cross section channel 20 laser machined in a substrate 22 by a laser beam with a square flat focal spot, one of which is shown 24. The over lapping squares, 26, represent successive positions of the laser focal spot as the channel 20 is machined. As in FIG. 1, the number of overlapping laser focal spot positions shown is schematic and may vary in actual practice depending upon laser repetition rate, laser pulse energy, pulse size, and other laser parameters. Note that since the laser energy is distributed evenly over a square focal spot, the calculated cumulative dose received by each point in the channel is equal as the laser spot moves along the desired machining path, causing the resulting channel to have a flat bottom with square edges. This is quite often desirable in LDA applications. Note that this analysis works with both pulsed and CW lasers.
Laser machining systems designed to machine certain features in electronic substrates, for example vias in multilayer electronic substrates have been described in the prior art. U.S. Pat. No. 5,798,927, APPARATUS AND METHOD FOR COORDINATING THE MOVEMENTS OF STAGES IN MULTI-STAGE MULTI-RATE POSITIONER SYSTEM, inventors Cutler, et. al. assigned to the assignee of the instant invention describes combining beam steering devices with high and low positioning rates and accuracies to quickly and accurately position a laser beam on a workpiece. U.S. Pat. No. 6,433,301 BEAM SHAPING AND PROJECTION IMAGING WITH SOLID STATE UV GAUSSIAN BEAM TO FORM VIAS, inventors Dunsky, et. al. describes using diffractive optical elements to form desired beam shapes for via drilling applications. FIG. 3 shows a diagram of a prior art laser machining system designed to machine vias in multilayer electronic substrates. A laser 30, which may be a pulsed, solid state UV laser emits, at the direction of the controller 32, laser pulses 34 which are shaped by the beam shaping optics 36 which may be holographic or diffractive, which are then steered by the beam steering optics 38, which may be multi-stage, at the direction of controller 32, to the scan optics 40 which may be an f-theta lens, onto the workpiece 42 which may be a multilayer electronic substrate, which is fixured on a motion control unit 44 which moves the workpiece 42 in relation to the laser pulses 34 at the direction of the controller 32 and in cooperation with the beam steering optics 36 to cause the laser pulses 34 to machine the desired feature in the workpiece 42. Exemplary systems employing these elements to machine features in electronic substrates are the Si5330 and the ICP5650 laser processing systems, manufactured by Electro Scientific Industries, Portland, Oreg., and the assignee of the instant invention.
FIG. 4 is a simulation of the result of using a prior art laser machining system as described in FIG. 3 to machine a curved channel in a substrate 50. A laser processing system (not shown) directs a series of square-flat laser pulses 52 onto a substrate 50 starting at pulse 54 and ending at pulse 56, following path 57. The cross sections 58, 62 and 66 of the resulting channel, taken along lines 60, 64 and 68, respectively, correspond to the cumulative laser radiation dose along lines 60, 64 and 68, respectively. Note that while cross sections 58 and 66 are acceptable, as the laser pulses 52 travel along path 57 the shape of the machined channel begins to deviate from the square-sided, flat-bottomed cross section 58 until it becomes as in cross section 62, then gradually changing back to cross section 66. This is an undesirable result. It is evident from these results that the prior art system does not achieve the desired topology of the machined channel by not properly controlling the cumulative laser radiation dose at each point in the feature as machining takes place.
What is needed then is a method and apparatus for achieving laser machined channels with a desired consistent size and depth in electronic substrates as the channel path changes direction and shape.